Calais
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Calais clear sky chart from 2005-11-06 to 2026-08-30.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	19% 9% 3 7% 4 4 4 4 5% 7% 35%	143919
	Night	19% 9% 3 7% 4 4 4 3 5% 6% 35%	138393
Transparency	Day	49% 4 11% 22% 13% 2	147436
	Night	49% 3 11% 23% 13% 2	142967
Seeing	Day	26% 17% 31% 19% 6% 1	18611
	Night	25% 15% 31% 23% 6%	26525

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	13% 12% 4 9% 6% 5% 5% 4 6% 6% 31%	4264
	11th to 20th	Day	17% 13% 3 9% 5% 3 3 2 4 4 36%	4650
	21st to end	Day	11% 8% 3 8% 5% 5% 5% 4 5% 7% 39%	5481
February	1st to 10th	Day	16% 12% 3 8% 5% 6% 4 3 5% 7% 32%	1890
	11th to 20th	Day	14% 11% 3 8% 5% 5% 4 4 5% 6% 35%	1854
	21st to end	Day	12% 12% 2 7% 5% 4 5% 4 5% 7% 37%	1730
March	1st to 10th	Day	25% 11% 3 8% 5% 4 4 3 4 5% 28%	4758
	11th to 20th	Day	22% 14% 3 6% 4 4 4 3 5% 7% 28%	5090
	21st to end	Day	27% 11% 4 8% 4 4 4 3 5% 5% 25%	5587
April	1st to 10th	Day	22% 12% 3 7% 4 4 4 3 4 6% 30%	2456
	11th to 20th	Day	22% 9% 3 6% 4 3 4 2 5% 6% 36%	2702
	21st to end	Day	24% 9% 2 6% 4 4 3 3 5% 5% 35%	2730
May	1st to 10th	Day	29% 10% 3 7% 4 4 4 3 4 6% 26%	7639
	11th to 20th	Day	20% 8% 3 7% 4 4 5% 4 6% 7% 34%	7780
	21st to end	Day	19% 9% 3 8% 5% 4 4 3 5% 7% 32%	9392
June	1st to 10th	Day	23% 8% 3 8% 4 6% 5% 3 5% 6% 30%	3150
	11th to 20th	Day	20% 8% 2 6% 4 3 3 3 5% 8% 37%	3150
	21st to end	Day	27% 7% 3 7% 5% 4 4 3 6% 6% 28%	3150
July	1st to 10th	Day	14% 7% 2 7% 5% 5% 5% 5% 7% 9% 35%	8880
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	8530
	21st to end	Day	16% 6% 2 6% 4 4 5% 4 6% 8% 39%	8636
August	1st to 10th	Day	21% 7% 3 7% 4 5% 4 3 5% 7% 34%	4365
	11th to 20th	Day	16% 7% 3 8% 5% 5% 5% 4 6% 8% 32%	4305
	21st to end	Day	12% 5% 2 5% 4 4 5% 5% 7% 8% 44%	4792
September	1st to 10th	Day	13% 4 1 5% 5% 4 3 3 5% 7% 51%	2345
	11th to 20th	Day	16% 6% 2 5% 3 3 3 3 4 6% 50%	2200
	21st to end	Day	19% 7% 2 5% 3 3 3 2 4 6% 46%	2200
October	1st to 10th	Day	18% 7% 2 6% 4 4 4 3 5% 7% 41%	2074
	11th to 20th	Day	20% 6% 2 5% 5% 4 4 3 4 8% 38%	2000
	21st to end	Day	22% 8% 3 6% 6% 4 4 4 4 5% 34%	2043
November	1st to 10th	Day	21% 7% 3 6% 4 4 4 3 5% 4 39%	1851
	11th to 20th	Day	20% 6% 2 6% 3 3 4 3 5% 7% 40%	1692
	21st to end	Day	20% 10% 3 8% 4 4 4 3 5% 5% 34%	1680
December	1st to 10th	Day	22% 10% 2 5% 3 3 4 4 6% 8% 33%	2632
	11th to 20th	Day	23% 9% 3 7% 4 4 3 2 3 4 38%	2688
	21st to end	Day	22% 10% 3 5% 3 4 4 3 5% 5% 35%	3553
January	1st to 10th	Night	14% 12% 4 9% 5% 5% 5% 3 5% 6% 32%	8635
	11th to 20th	Night	19% 12% 4 9% 5% 4 4 3 5% 6% 30%	8235
	21st to end	Night	9% 11% 3 7% 4 4 4 4 5% 6% 42%	8699
February	1st to 10th	Night	15% 11% 3 7% 5% 4 5% 4 6% 6% 33%	2730
	11th to 20th	Night	15% 11% 3 9% 6% 4 5% 3 5% 5% 33%	2648
	21st to end	Night	16% 11% 3 6% 5% 4 4 3 5% 5% 37%	2076
March	1st to 10th	Night	26% 14% 4 8% 4 5% 3 2 3 4 27%	5302
	11th to 20th	Night	21% 13% 3 6% 4 4 5% 4 5% 6% 29%	5089
	21st to end	Night	31% 9% 2 6% 4 4 3 3 4 5% 29%	5587
April	1st to 10th	Night	24% 12% 3 6% 5% 4 3 3 3 5% 32%	2161
	11th to 20th	Night	25% 10% 3 5% 3 4 5% 2 5% 5% 33%	1868
	21st to end	Night	25% 8% 3 6% 4 4 4 3 5% 6% 31%	1890
May	1st to 10th	Night	30% 9% 3 6% 3 3 3 3 4 6% 30%	5259
	11th to 20th	Night	21% 9% 2 6% 5% 4 5% 4 5% 7% 31%	5073
	21st to end	Night	17% 9% 3 7% 5% 4 5% 4 5% 7% 33%	4857
June	1st to 10th	Night	25% 7% 2 6% 4 4 5% 4 6% 7% 30%	1470
	11th to 20th	Night	19% 7% 3 6% 5% 4 4 4 6% 7% 34%	1470
	21st to end	Night	26% 7% 2 6% 5% 4 4 3 5% 7% 29%	1470
July	1st to 10th	Night	14% 7% 3 7% 6% 5% 6% 6% 6% 7% 33%	4110
	11th to 20th	Night	18% 6% 2 6% 4 3 4 4 6% 8% 38%	4426
	21st to end	Night	15% 6% 2 5% 4 3 4 4 6% 8% 42%	5381
August	1st to 10th	Night	21% 9% 3 7% 4 5% 4 3 5% 6% 32%	3005
	11th to 20th	Night	15% 9% 3 8% 5% 4 5% 4 6% 8% 34%	2962
	21st to end	Night	11% 6% 2 6% 4 4 5% 3 5% 8% 46%	3305
September	1st to 10th	Night	16% 5% 2 4 5% 4 3 3 5% 7% 47%	2049
	11th to 20th	Night	17% 6% 2 4 2 3 3 3 5% 6% 50%	2200
	21st to end	Night	19% 6% 2 5% 3 3 3 3 5% 7% 45%	2200
October	1st to 10th	Night	20% 6% 2 5% 3 4 4 3 5% 7% 43%	2325
	11th to 20th	Night	19% 7% 3 6% 4 3 3 3 5% 6% 41%	2400
	21st to end	Night	23% 8% 3 7% 4 4 3 3 5% 6% 35%	2795
November	1st to 10th	Night	20% 7% 2 6% 4 4 4 4 5% 6% 39%	2667
	11th to 20th	Night	19% 7% 2 6% 4 4 4 3 5% 6% 38%	2925
	21st to end	Night	21% 8% 2 7% 4 4 4 4 5% 7% 35%	2940
December	1st to 10th	Night	18% 9% 3 7% 4 3 4 3 5% 6% 38%	4776
	11th to 20th	Night	23% 9% 2 5% 3 4 4 3 4 4 38%	5741
	21st to end	Night	21% 11% 3 7% 3 4 4 3 4 5% 34%	7667

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	53% 2 12% 23% 8% 1	4265
	11th to 20th	Day	53% 3 8% 24% 11% 2	4680
	21st to end	Day	45% 2 8% 22% 19% 3	5481
February	1st to 10th	Day	53% 3 10% 19% 13% 2	1890
	11th to 20th	Day	50% 3 11% 21% 13% 2	1862
	21st to end	Day	46% 4 11% 23% 12% 5%	1730
March	1st to 10th	Day	60% 2 8% 13% 14% 4	4762
	11th to 20th	Day	57% 4 12% 17% 10% 1	5090
	21st to end	Day	61% 3 4 17% 13% 2	5587
April	1st to 10th	Day	55% 4 8% 15% 15% 3	2456
	11th to 20th	Day	49% 4 8% 23% 14% 2	2702
	21st to end	Day	50% 4 7% 20% 14% 5%	2730
May	1st to 10th	Day	60% 3 9% 18% 9% 1	7651
	11th to 20th	Day	50% 3 13% 23% 9% 2	7784
	21st to end	Day	52% 3 11% 18% 14% 2	9393
June	1st to 10th	Day	54% 4 10% 20% 11% 1	3150
	11th to 20th	Day	46% 4 11% 22% 16% 2	3150
	21st to end	Day	56% 5% 10% 17% 11% 2	3150
July	1st to 10th	Day	44% 5% 14% 25% 12% 1	8805
	11th to 20th	Day	41% 5% 13% 23% 16% 2	8450
	21st to end	Day	43% 5% 12% 28% 13% 1	8446
August	1st to 10th	Day	50% 5% 11% 23% 10% 1	4352
	11th to 20th	Day	49% 4 15% 22% 9%	4344
	21st to end	Day	35% 4 13% 28% 20% 1	4792
September	1st to 10th	Day	33% 4 11% 31% 19% 1	2345
	11th to 20th	Day	36% 3 9% 27% 21% 3	2200
	21st to end	Day	41% 3 8% 29% 18% 1	2200
October	1st to 10th	Day	49% 4 11% 25% 11% 1	3373
	11th to 20th	Day	39% 3 10% 34% 14%	3240
	21st to end	Day	37% 3 16% 24% 18% 1	3280
November	1st to 10th	Day	48% 4 11% 22% 15% 1	1851
	11th to 20th	Day	44% 4 12% 22% 15% 2	1692
	21st to end	Day	52% 4 13% 22% 8% 1	1680
December	1st to 10th	Day	48% 4 15% 23% 10%	2632
	11th to 20th	Day	53% 3 11% 17% 12% 5%	2688
	21st to end	Day	50% 4 11% 20% 13% 3	3553
January	1st to 10th	Night	53% 2 10% 24% 10% 1	8635
	11th to 20th	Night	56% 3 10% 21% 10% 1	8280
	21st to end	Night	43% 2 9% 24% 17% 4	8699
February	1st to 10th	Night	50% 4 11% 20% 13% 2	2730
	11th to 20th	Night	52% 4 8% 21% 13% 2	2651
	21st to end	Night	49% 2 9% 22% 15% 3	2076
March	1st to 10th	Night	63% 2 5% 12% 13% 4	5311
	11th to 20th	Night	56% 3 15% 17% 7% 1	5090
	21st to end	Night	58% 2 5% 19% 14% 1	5587
April	1st to 10th	Night	56% 4 7% 16% 14% 4	2161
	11th to 20th	Night	53% 3 8% 19% 16% 2	1868
	21st to end	Night	54% 3 8% 18% 15% 2	1890
May	1st to 10th	Night	57% 2 13% 16% 10% 1	5265
	11th to 20th	Night	53% 3 12% 22% 9% 1	5074
	21st to end	Night	51% 4 12% 21% 11% 1	4858
June	1st to 10th	Night	53% 4 10% 23% 9% 1	1470
	11th to 20th	Night	48% 4 11% 25% 11% 2	1470
	21st to end	Night	55% 4 10% 20% 8% 3	1470
July	1st to 10th	Night	49% 4 14% 23% 9%	4075
	11th to 20th	Night	44% 4 13% 28% 11%	4383
	21st to end	Night	41% 4 13% 27% 14% 1	5258
August	1st to 10th	Night	54% 5% 12% 20% 9%	2993
	11th to 20th	Night	47% 5% 16% 24% 8%	2989
	21st to end	Night	35% 5% 14% 27% 18% 2	3305
September	1st to 10th	Night	38% 4 11% 31% 16% 1	2049
	11th to 20th	Night	36% 3 12% 28% 20% 2	2200
	21st to end	Night	39% 4 9% 31% 16% 1	2200
October	1st to 10th	Night	45% 3 11% 28% 12% 1	3806
	11th to 20th	Night	38% 2 10% 30% 19%	3899
	21st to end	Night	43% 2 12% 27% 14% 2	4508
November	1st to 10th	Night	46% 3 13% 24% 14% 1	2667
	11th to 20th	Night	47% 4 10% 23% 14% 1	2925
	21st to end	Night	49% 5% 13% 21% 11% 1	2940
December	1st to 10th	Night	47% 3 11% 29% 10%	4776
	11th to 20th	Night	49% 3 11% 21% 15% 2	5742
	21st to end	Night	53% 3 10% 19% 13% 3	7667

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	30% 23% 32% 13% 1	220
	11th to 20th	Day	35% 25% 29% 9% 2 1	220
	21st to end	Day	23% 22% 34% 18% 3	367
February	1st to 10th	Day	27% 22% 30% 17% 4	430
	11th to 20th	Day	24% 28% 34% 10% 3	432
	21st to end	Day	26% 26% 30% 15% 3	446
March	1st to 10th	Day	29% 26% 29% 13% 2	540
	11th to 20th	Day	30% 23% 31% 11% 4 1	540
	21st to end	Day	35% 23% 25% 11% 4 2	594
April	1st to 10th	Day	36% 23% 28% 9% 1 1	540
	11th to 20th	Day	34% 21% 30% 12% 3 1	532
	21st to end	Day	30% 20% 33% 13% 4 1	540
May	1st to 10th	Day	33% 19% 30% 15% 3	543
	11th to 20th	Day	25% 20% 33% 17% 4 1	618
	21st to end	Day	27% 17% 32% 17% 6%	845
June	1st to 10th	Day	26% 16% 32% 20% 5% 1	780
	11th to 20th	Day	25% 14% 31% 21% 7% 2	780
	21st to end	Day	32% 9% 31% 19% 8% 1	780
July	1st to 10th	Day	20% 9% 36% 25% 9% 1	770
	11th to 20th	Day	22% 10% 33% 24% 9% 1	653
	21st to end	Day	19% 10% 35% 29% 6% 1	620
August	1st to 10th	Day	17% 10% 34% 28% 10% 1	566
	11th to 20th	Day	18% 8% 33% 28% 10% 2	559
	21st to end	Day	19% 11% 32% 26% 9% 3	619
September	1st to 10th	Day	14% 12% 36% 28% 8% 2	530
	11th to 20th	Day	23% 12% 28% 20% 12% 4	530
	21st to end	Day	19% 16% 31% 22% 8% 3	530
October	1st to 10th	Day	19% 15% 29% 25% 10% 3	460
	11th to 20th	Day	23% 17% 29% 19% 10% 2	420
	21st to end	Day	27% 17% 33% 17% 5%	462
November	1st to 10th	Day	29% 19% 28% 19% 5% 1	425
	11th to 20th	Day	26% 24% 29% 15% 5%	430
	21st to end	Day	30% 21% 27% 18% 3	430
December	1st to 10th	Day	30% 20% 30% 15% 5%	398
	11th to 20th	Day	36% 29% 24% 9% 2	220
	21st to end	Day	31% 28% 26% 10% 4	242
January	1st to 10th	Night	26% 22% 29% 20% 3	914
	11th to 20th	Night	28% 19% 30% 19% 3	840
	21st to end	Night	18% 24% 37% 17% 4	924
February	1st to 10th	Night	25% 20% 32% 20% 3	840
	11th to 20th	Night	25% 26% 30% 16% 3 1	823
	21st to end	Night	24% 21% 33% 18% 4	692
March	1st to 10th	Night	26% 21% 34% 18% 2	840
	11th to 20th	Night	26% 16% 32% 20% 5%	840
	21st to end	Night	28% 16% 32% 19% 5%	924
April	1st to 10th	Night	34% 17% 31% 15% 2	787
	11th to 20th	Night	30% 16% 32% 18% 4	624
	21st to end	Night	33% 11% 33% 18% 6%	630
May	1st to 10th	Night	28% 13% 30% 23% 5%	630
	11th to 20th	Night	29% 13% 31% 22% 5%	622
	21st to end	Night	23% 12% 37% 23% 5% 1	587
June	1st to 10th	Night	27% 6% 33% 29% 5%	420
	11th to 20th	Night	23% 8% 33% 28% 8% 1	420
	21st to end	Night	28% 4 30% 32% 6%	420
July	1st to 10th	Night	18% 5% 40% 29% 8%	410
	11th to 20th	Night	22% 7% 25% 35% 11%	408
	21st to end	Night	20% 4 30% 35% 10% 1	564
August	1st to 10th	Night	18% 5% 31% 36% 10%	628
	11th to 20th	Night	19% 5% 29% 36% 11%	622
	21st to end	Night	17% 7% 29% 34% 13% 1	688
September	1st to 10th	Night	18% 4 37% 29% 12% 1	690
	11th to 20th	Night	20% 9% 27% 29% 13% 2	800
	21st to end	Night	21% 10% 29% 27% 12% 1	800
October	1st to 10th	Night	22% 8% 29% 29% 12%	800
	11th to 20th	Night	23% 12% 32% 23% 10% 1	800
	21st to end	Night	27% 16% 30% 21% 6%	880
November	1st to 10th	Night	28% 14% 30% 22% 6% 1	820
	11th to 20th	Night	26% 18% 29% 20% 7% 1	943
	21st to end	Night	29% 18% 29% 19% 5% 1	950
December	1st to 10th	Night	32% 15% 30% 19% 4	950
	11th to 20th	Night	29% 24% 29% 15% 2	950
	21st to end	Night	30% 22% 29% 16% 4	1045

Page generated at 2026-08-27 04:07:15. Version ac07

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